This invention relates to the injection molding of plastics, and more particularly to a method and an apparatus for sequentially injection molding a plurality of thermoplastic parts in a single mold.
Injection molding machines are widely used in the production of thermoplastic parts. Such machines comprise a plasticizing chamber in which plastic molding powders are heated to form a viscous melt, a nozzle communicating between said chamber and a mold held between a pair of platens, and a piston or screw for applying pressure to the melt to force it through the nozzle and into the cavity of the mold.
A very large force must be exerted against the platens to hold the mold in place while the plastic is being injected therein. For example, for a medium size part such as a 50 centimeter diameter fan blade, this force may be about 180 to 360 tonnes (200 to 400 tons).
It would be desirable, from the standpoint of productivity, to provide a machine which could produce multiple parts rather than only a single part in the mold before the mold is opened and the part removed therefrom. However, if two parts, for example, are formed simultaneously, it has been found that the force required to prevent separation of the mold parting line is approximately twice that for a single part. Hence, a much larger and more expensive molding machine is required.
It is an object of this invention to provide a method and an apparatus for producing multiple thermoplastic parts without any increase in the force required to prevent mold separation.